Am 29.03.2019 um 18:30 hat Max Reitz geschrieben: > On 29.03.19 18:24, Kevin Wolf wrote: > > Am 29.03.2019 um 18:15 hat Max Reitz geschrieben: > >> On 29.03.19 12:04, Vladimir Sementsov-Ogievskiy wrote: > >>> bdrv_replace_child() calls bdrv_check_perm() with error_abort on > >>> loosening permissions. However file-locking operations may fail even > >>> in this case, for example on NFS. And this leads to Qemu crash. > >>> > >>> Let's avoid such errors. Note, that we ignore such things anyway on > >>> permission update commit and abort. > >>> > >>> Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]> > >>> --- > >>> block/file-posix.c | 12 ++++++++++++ > >>> 1 file changed, 12 insertions(+) > >>> > >>> diff --git a/block/file-posix.c b/block/file-posix.c > >>> index db4cccbe51..1cf4ee49eb 100644 > >>> --- a/block/file-posix.c > >>> +++ b/block/file-posix.c > >>> @@ -815,6 +815,18 @@ static int raw_handle_perm_lock(BlockDriverState *bs, > >>> > >>> switch (op) { > >>> case RAW_PL_PREPARE: > >>> + if ((s->perm | new_perm) == s->perm && > >>> + (s->shared_perm & new_shared) == s->shared_perm) > >>> + { > >>> + /* > >>> + * We are going to unlock bytes, it should not fail. If it > >>> fail due > >>> + * to some fs-dependent permission-unrelated reasons (which > >>> occurs > >>> + * sometimes on NFS and leads to abort in > >>> bdrv_replace_child) we > >>> + * can't prevent such errors by any check here. And we > >>> ignore them > >>> + * anyway in ABORT and COMMIT. > >>> + */ > >>> + return 0; > >>> + } > >>> ret = raw_apply_lock_bytes(s, s->fd, s->perm | new_perm, > >>> ~s->shared_perm | ~new_shared, > >>> false, errp); > >> > >> Help me understand the exact issue, please. I understand that there are > >> operations like bdrv_replace_child() that pass &error_abort to > >> bdrv_check_perm() because they just loosen the permissions, so it should > >> not fail. > >> > >> However, if the whole effect really would be to loosen permissions, > >> raw_apply_lock_bytes() wouldn't have failed here in PREPARE anyway: > >> @unlock is passed as false, so no bytes will be unlocked. And if > >> permissions are just loosened (as your condition checks), it should not > >> lock any bytes. > >> > >> So why does it attempt lock any bytes in the first place? There must be > >> some discrepancy between s->perm and s->locked_perm, or ~s->shared_perm > >> and s->locked_shared_perm. How does that occur? > > > > I suppose raw_check_lock_bytes() is what is failing, not > > raw_apply_lock_bytes(). > > Hm, maybe in Vladimir's case, but not in e.g. > https://bugzilla.redhat.com/show_bug.cgi?id=1652572 .
This is reported against 3.0, which didn't avoid re-locking permissions that we already hold, so there raw_apply_lock_bytes() can still fail. Kevin
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